The present invention concerns the detoxication of aqueous solutions contaminated with hexavalent chromium by electrolytic reduction to trivalent chromium.
Various industrial operations such as electroplating, generate waste solutions containing significant amounts of hexavalent chromium (Cr+6). Cr+6 is known to be carcinogenic, and highly toxic to humans, animals and plants and it is therefore of paramount importance to ensure that no Cr+6 penetrates into the soil and ground water. According to current standards, industrial aqueous waste solutions discharged into the ground or the sea may not contain more than 0.1 mg/liter of Cr+6.
U.S. Pat. No. 3,679,557 describes an electrolytic reactor for continuously reducing Cr+6 in aqueous solution to Cr+3, in which the cathode is in form of a bed of carbon particles. The operation of this reactor is very slow and according to one typical example it took 208 hours to treat 300 liters of a solution for reducing its Cr+6 content from 13.5 ppm to 0.05 ppm.
U.S. Pat. No. 4,436,601 describes an electrolytic reactor for the removal of metals from aqueous solutions comprising a plurality of electrically energized anodes and cathodes with each anode having openings to permit passage of the waste water to be electrolyzed, each cathode being in the form of a metalized reticulate organic polymer foam. The metal used for the metalization of the cathodes must be inert under the reaction conditions and copper, nickel, silver and gold are mentioned specifically. In the course of operation, the metals to be removed are precipitated on the cathodes and in consequence the cathodes have only a limited lifetime. The reduction of Cr+6 to Cr+3 is not mentioned specifically in the disclosure.
U.S. Pat. No. 5,326,439 discloses a method for the removal of Cr+6 from ground water by a chemical reaction with Fe(OH)2 inside an aquifer. In accordance with the disclosure an electrolytic reactor containing iron electrodes generates an aqueous suspension of ferrous hydroxide which is flown into the ground water where the reaction takes place leading to the precipitation of Cr(OH)3 and possibly other matter. The so treated ground water is withdrawn from its aquifer and all precipitated matter is filtered off.
It is the object of the present invention to provide a new electrolytic reactor for the effective and efficient reduction of Cr+6 to Cr+3 in aqueous solution.
It is a further object of the present invention to provide electrode means for use in an electrolytic reactor and a method for the production of such electrode means.
The electrolytic reactor with which the present invention is concerned is of the throughflow type in which the aqueous solution to be treated flows continuously across a reactor and the electrolytic treatment occurs in the course of such throughflow. Such a type of reactor will be referred to hereinafter for short as xe2x80x9cdynamic electrolytic reactorxe2x80x9d.
In accordance with the present invention there is provided a dynamic electrolytic reactor for the reduction of Cr+6 to Cr+3 in an aqueous solution comprising a vessel with liquid inlet and outlet and holding inside at least one pair of liquid permeable anode and cathode connectable each to an electric current supply, characterized in that said cathode is three-dimensional and comprises a substrate of flexible porous material whose inner and outer surfaces are coated with an electrically conductive carbon black powder embedded in a binder.
The cathode in an electrolytic reactor according to the invention may be a single block or consists of several juxtaposed layers.
The liquid permeable anode in a reactor according to the invention may, for example, be in form of wire net or a plate having a plurality of holes.
The substrate of the flexible cathode in a dynamic electrolytic reactor according to the invention may, for example be of a synthetic material of the reticulated foam type. Due to the flexibility of the substrate, it is possible to manufacture large bodies of cathode material and cut them to size. As the flexible material is readily adaptable to the inner shape of the reactor there is no need for a very high degree of precision when cutting the cathode to size.
The invention further provides a composite material for making therefrom cathodes for reactors according to the invention which body comprises a flexible porous substrate whose outer and inner surfaces are coated with electrically conductive carbon black powder embedded in a binder. For assembling dynamic electrolytic reactors according to the invention cathode bodies of required size are cut out from the said composite.
The porosity of the porous material used for the purpose of the present invention is preferably within the range from about 5 to about 20 pores per lineal inch (ppi) according to the Mil-B-830548 standard and determined by Air Pressure Drop Test. If the porosity exceeds significantly 20 ppi the individual pores will be too small and offer too large a resistance to the through flowing aqueous solution.
Typical examples of porous materials usable as substrates for the purposes of the present invention are reticulated polyurethane foams such as SAFOM(copyright) or BULPREN(copyright) both manufactured by Reticel, Belgium.
The electrically conductive carbon black powder suitable for the purpose of the present invention is commercially available and may, for example, be that sold under the tradename Printex L(trademark) by Degussa, Germany.
For the preparation of a composite body according to the invention a coating mixture is prepared by mixing electrically conductive carbon black powder, a binder and a solvent, and preferably milling the resulting liquid mixture, e.g. in a ball mill. The resulting coating mixture is pored into a plating bath and a body of flexible synthetic porous material (reticulated foam) is dipped into the coating mixture until fully wetted therewith. The fully wetted porous material is squeezed to remove excess liquid and is then dried. If desired, the coating operation may be repeated twice or more in order to achieve a desired electrical conductivity.